Hepatitis B surface antigen (HBsAg) is purified and its polypeptides are compared with respect to antigenicity and immunogenicity, and whether host proteins are associated with them. The prognostic significance of hepatitis B virions (HBV), the e antigen, and the HVB-associated DNA polymerase is being assessed by identifying them in the serum of hepatitis B patients with different clinical manifestations of hepatitis B infection, such as persistent asymptomatic hepatitis or chronic progressive hepatitis. Hepaptitis A virus (HAV) is being detected in fecal specimens by immune electron microscopy (IEM) and immune adherence hemaglutination (IAHA). Hepatitis A antibody (HAAb) is identified by these techniques with the aid of HAV preparations. HAV and HAAb from different persons are being compared. Attempts to propagate HAV and HBV are being made.